Centralize terminal history/scrollback/buffer limits into config (no behavior change)

[aakhter]

What

Introduces src/config/terminal-history.ts as the single source of truth for terminal history retention — terminal scrollback lines, tmux history-limit, and the server PTY buffer byte caps that were previously scattered as hardcoded literals across buffer-limits.ts, tmux-manager.ts, and session.ts.

Each value becomes overridable (env var or the settings object) and is bounds-clamped via a small pure resolveTerminalHistoryConfig().

Behavior-neutral

Defaults intentionally match the prior hardcoded values, so this PR changes no runtime behavior on its own:

value	before	this PR
tmux history-limit	50,000	50,000
terminal scrollback lines	50,000 (DEFAULT_SCROLLBACK)	50,000
PTY buffer max	2 MB	2 MB
PTY buffer trim	1.5 MB	1.5 MB

A follow-up PR (stacked on this one) raises these defaults; keeping the mechanism and the policy change separate so each can be reviewed on its own merits.

Wiring

• buffer-limits.ts now sources MAX_TERMINAL_BUFFER_SIZE / TRIM_TERMINAL_TO from the resolver (env-override semantics preserved).
• tmux-manager.ts uses DEFAULT_TMUX_HISTORY_LIMIT in place of the hardcoded history-limit 50000, and gains a setHistoryLimit() (mux-interface + impl) so a settings change applies to live sessions; also re-applied on respawnPane so the limit survives a respawn.
• server.ts exposes getTerminalHistoryConfig() on the route ctx; system-routes.ts applies a changed tmuxHistoryLimit live.
• session.ts threads a per-session tmuxHistoryLimit into the tmux spawn calls.
• schemas.ts adds 4 optional, bounds-clamped settings keys (terminalScrollbackLines, tmuxHistoryLimit, terminalBufferMaxBytes, terminalBufferTrimBytes) with a trim <= max cross-check.

Tests

New test/terminal-history.test.ts (resolver defaults / clamping / trim<=max / non-number fallback) and test/terminal-history-schema.test.ts (settings-schema validation). tsc --noEmit, lint, the focused tests, and npm run build all pass.