fix(terminal): read the colour env from the registry on every local spawn path

buildClaudeEnv(), the direct-PTY fallback taken when mux creation fails, now
reads getCli('claude').env and applies its unset and exports lists. It used to
delete COLORTERM and CLAUDECODE from a hand-maintained list of its own, which
left it contradicting the registry entry that the tmux pane and the attach
client both read. An engine value needing a mux name has nothing to resolve
against on this path, so it is skipped rather than guessed.

Claude no longer unsets NO_COLOR. The invisible-background bug does not need
it, and unsetting it overrides a preference the user set deliberately, so a
user who exports NO_COLOR globally keeps monochrome panes. The other seven
truecolor CLIs still unset it; that inconsistency is intentional and the
comment on the entry says so.

The invariants doc gains a Terminal colour env paragraph under Session launch
modes, where a reader looking up Claude will find it — the previous sentence
sat under a heading that lists only the non-Claude CLIs. It now says the lists
are the stock catalog and a clis.json override replaces them wholesale, and
that the declarations reach the tmux pane, its attach client and the direct
PTY but not a remote pane, whose command carries no env exports at all. Docker
hands COLORTERM=truecolor to every mode, including the two the registry says
must unset it.

The changeset named six peer CLIs and there are seven: deepseek also exports
truecolor. A test beside the existing OpenCode assertion pins the new
behaviour, so a future registry edit cannot make the backgrounds vanish again
in silence.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/config/cli-registry/stock.ts

@@ -174,17 +174,19 @@ const CLAUDE: CliEntry = {
     legacyConfigAliases: { resumeId: 'resumeSessionId' },
   },
   env: {
-    // Claude joins the truecolor list with COLORTERM=truecolor and NO_COLOR unset.
+    // Claude asks for truecolor, like every CLI here except `shell` and `opencode`.
     // tmux hands the pane TERM=screen, which supports-color reads as 16 colors, and
     // Claude then quantizes every RGB color its theme asks for down to that palette.
     // Each dark background lands on ESC[40m, the terminal's own black, so the block
     // Claude draws behind the user's own messages renders invisible. PR #3 unset
     // COLORTERM here against xterm.js#484, which xterm.js had already closed in 2019,
     // and Codeman now ships @xterm/xterm 6 and sets `terminal-overrides *:Tc` itself.
+    // The other truecolor CLIs also unset NO_COLOR. Claude does not, so a user who
+    // exports NO_COLOR globally keeps the monochrome panes they asked for.
     // CLAUDECODE stays unset, because Claude reads it as a signal that it is running
     // nested inside itself.
     exports: [{ name: 'COLORTERM', value: 'truecolor' }],
-    unset: ['CLAUDECODE', 'NO_COLOR'],
+    unset: ['CLAUDECODE'],
     tmuxSetenvKeys: [],
     dockerExecEnvNames: [],
     allowedPrefixes: ['CLAUDE_CODE_'],