fix(custom-model): isolate Claude config dir and inject real context length

Addresses two live-validation findings on the Run-menu custom-model picker:

1. Both claude.ai and ANTHROPIC_API_KEY set warning. Claude Code still
   coexists an OAuth login with an injected ANTHROPIC_API_KEY in the same
   config directory and warns about it (confirmed cosmetic - the API key
   wins for actual requests, verified via a real session's own API Usage
   Billing line). A custom-model claude session now gets an isolated
   CLAUDE_CONFIG_DIR (registry-declared via a new configDirVar field, empty,
   no files written into it) so there is nothing to conflict with. projects
   is symlinked (junction on Windows) back into the real config dir so the
   response viewer, subagent windows and Read My Mind keep working for that
   session, best-effort.

2. Context-window overflow. Claude Code assumes a large default context
   window for a model id it doesn't recognise and never compacts, so a
   custom endpoint's real, much smaller context (verified live: a 400
   exceeding a 16384-token llama-swap model with a stock ~33.7K-token system
   prompt) silently overflows. Discovery now also learns each model's real
   context length from llama.cpp/llama-swap's GET /props?model=<id> (n_ctx),
   but ONLY for a model llama-swap's own /v1/models response already marks
   status.value === 'loaded' - never an unloaded one, since llama-swap
   treats ?model= as a routing hint and probing an unloaded model risks
   triggering an actual, slow, GPU-swapping load as a side effect of
   read-only discovery. A server with no status field at all gets no
   enrichment rather than a guess; a model not probed this round keeps its
   previously-learned value until it disappears from the list entirely.
   Stored per model (CustomModelHost.modelContextLengths) and applied via a
   new contextLengthVar registry field, set to
   CLAUDE_CODE_MAX_CONTEXT_TOKENS for claude.

Both new fields live on the existing env-kind customModelInjection
capability shape, declared only on claude's registry entry - every other
CLI's injection is unaffected (pinned by test).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

docs/custom-model-endpoints.md

@@ -66,6 +66,23 @@ configured, `PUT`/`DELETE /api/model-endpoints/:id` update or remove one.
 Endpoint management is admin-only in multi-user mode, same as remote/docker
 hosts — these are machine-level infra, not per-user settings.
 
+**Context length is discovered too, opportunistically and safely.** The plain
+`GET /v1/models` response has no context-window field, but llama.cpp's
+llama-swap-proxied `GET /props?model=<id>` does (`n_ctx`). Discovery only ever
+calls it for a model llama-swap's own response already reports
+`status.value === "loaded"` for — never for an unloaded one, because
+llama-swap treats `?model=` as a routing hint and asking about a model that
+isn't loaded risks triggering an actual (slow, GPU-swapping) load as a side
+effect of what should be read-only discovery. A server with no `status` field
+on any entry at all (not llama-swap) gets no context-length enrichment,
+rather than guessing. A model's previously-learned context length survives a
+later cycle where it wasn't the loaded one; it's dropped only once the model
+disappears from the endpoint's list entirely. Stored per model in
+`modelContextLengths` and applied automatically (see "Applying a model to a
+session" below) so a CLI that would otherwise assume a large default context
+window for an unrecognized model id stops silently overflowing a much
+smaller real one.
+
 `defaultModelId` names which discovered model the picker pre-marks for that
 endpoint — the settings panel's Edit form exposes it as a select populated
 from the endpoint's own discovered `models`, and the route refuses a value
@@ -143,6 +160,34 @@ since for those three the config file alone does not switch the model.
 reattaches the durable remote/in-container tmux rather than relaunching the
 agent, so the selection would report success and change nothing.
 
+**Claude gets two more env vars when known/applicable, both declared on its
+registry entry (`contextLengthVar`/`configDirVar`), not hardcoded here:**
+
+- `CLAUDE_CODE_MAX_CONTEXT_TOKENS` is set to `modelId`'s discovered context
+  length (see the discovery section above) whenever one is known. Without
+  it, Claude Code assumes a large (200k) window for any unrecognized custom
+  model id and never compacts, which reliably overflows a much smaller real
+  local context — confirmed live: a stock ~33.7K-token system prompt against
+  a 16384-token llama-swap model failed with `exceeds the available context
+  size`. No entry for the model in `modelContextLengths` means the var is
+  simply omitted, never a guess.
+- `CLAUDE_CONFIG_DIR` is pointed at the same isolated per-session directory
+  the `configDir`-kind CLIs use (empty, no files written into it), so the
+  injected `ANTHROPIC_API_KEY` never shares a directory with a stored
+  claude.ai OAuth login. Claude Code still prints "Both claude.ai and
+  ANTHROPIC_API_KEY set" when the two coexist in the same config directory —
+  cosmetic (confirmed live: the API key wins for actual requests either way,
+  visible in the terminal's own `API Usage Billing` line) but worth
+  eliminating rather than living with. The directory's `projects`
+  subdirectory is symlinked (a junction on Windows) back to the real
+  `~/.claude/projects` so the response viewer, subagent windows and Read My
+  Mind keep working for that session — the same trade-off and fix documented
+  for a manually-set `CLAUDE_CONFIG_DIR` in
+  [`docs/wiki/Agent-CLIs.md`](wiki/Agent-CLIs.md), just applied
+  automatically here. Best-effort: a platform that refuses the symlink keeps
+  the pre-existing blind-response-viewer side effect rather than failing the
+  whole custom-model apply over it.
+
 Clear back to the harness's native cloud default with:
 
 ```bash