Reserve built-in slash-command names from prompt templates

[baixinghao]

Summary

Prompt templates are expanded before slash-command dispatch in CodingSession.handle_command(), so a template named new.md shadowed the built-in /new command (and likewise for every other built-in). Only prompts.md, skills.md, tools.md, and reload.md were previously reserved via a hard-coded list that could drift from the command registry.

This PR reserves every built-in command name and alias at resource-loading time: the reserved set is derived from create_default_command_registry() so it cannot drift, matching is case-insensitive on the filename stem, and each rejected template emits a resource diagnostic naming the file and the conflicting command (e.g. prompt template name is reserved by the built-in /exit command; template ignored). Non-conflicting templates keep the existing expansion-first behavior, at startup and after /reload.

Command-first dispatch was deliberately avoided: it would silently turn an existing new.md from a prompt expansion into a session reset. Load-time reservation makes the collision visible and deterministic instead.

Fixes #460.

Supersedes #468 (stale for ~3 months and conflicting with current main).

Changes

• src/tau_coding/prompt_templates.py: replace the hard-coded _RESERVED_TEMPLATE_NAMES frozenset with a cached builtin_prompt_template_reservations() mapping (stem -> "/command") derived from create_default_command_registry() (names and aliases, case-folded). is_prompt_template_candidate() and the directory loader both consult it; the loader reports the exact conflicting invocation in the diagnostic. The registry import is lazy because tau_coding.commands already imports PromptTemplate from this module.
• tests/test_prompt_templates.py: extend the reserved-name parametrization to new, quit, exit (and reload), keep the uppercase-filename case-insensitivity coverage, and add a drift guard asserting the reservation mapping covers every registry command name and alias.
• tests/test_coding_session.py: add session-level tests proving /new still requests a new session and /quit//exit still exit while new.md, quit.md, and exit.md are rejected with diagnostics; that a non-conflicting template keeps expanding; and that the behavior holds after /reload. Since TUI autocomplete and the /prompts picker consume session.prompt_templates, a rejected template can never be advertised.
• website/content/guides/skills-and-prompts.md: document the unified reservation policy (all built-in names and aliases, case-insensitive) and the migration path (new.md → rename to new-feature.md, invoke with /new-feature).
• dev-notes/reserved-prompt-template-names.md: beginner-friendly note covering the resolution order, why silent command-first dispatch was avoided, why extension commands are out of scope (they are dynamic across /reload), and how to validate.

Tests

• uv run pytest tests/test_prompt_templates.py tests/test_coding_session.py tests/test_commands.py tests/test_tui_autocomplete.py tests/test_project_trust.py — 333 passed, 5 skipped, 4 failed. The 4 failures (test_prompt_logs_unexpected_agent_call_exception, test_session_loads_tau_native_system_prompt_files, test_store_round_trip_is_sorted_and_nearest_decision_wins, test_tui_trust_modal_shows_boundary_parent_and_keyboard_cancel) reproduce identically on the clean base commit on Windows (verified via git stash); they are pre-existing and unrelated.
• uv run pytest ... -k reserved — 13 passed (loader-level parametrized cases for 7 reserved stems including the exit alias, the registry drift guard, and 5 session-level startup//reload tests).
• uv run ruff check . — all checks passed.
• uv run ruff format --check . — 199 files already formatted.
• uv run mypy — no new errors; the 14 reported errors are pre-existing Windows-only issues in tau_agent/session/storage.py and tau_coding/project_trust.py (msvcrt/fcntl platform APIs), and src/tau_coding/prompt_templates.py itself is clean.