feat(cli): add self update command

[pesap]

Closes #379

Summary

Adds a draft implementation of torc self update [TARGET_VERSION] under the new self
namespace. The command is local-only, uses axoupdater receipts to guard eligible standalone
installer updates, supports --token/GITHUB_TOKEN, and gives install-source guidance when the
current executable cannot be self-updated.

Draft PR: not ready to merge until maintainer review and forge CI are complete.

Acceptance criteria

• Addressed — Adds torc self update [TARGET_VERSION] under a new self namespace.
  Evidence: src/cli.rs wires SelfCommand as name = "self"; cargo +1.95.0 run --quiet --bin torc -- self update --help prints Usage: torc self update [OPTIONS] [TARGET_VERSION].
• Addressed — Command is local-only: it does not require a Torc server, does not run server
  version checks, and ignores --standalone startup. Evidence: src/main.rs excludes
  Commands::SelfCommand { .. } from requires_server; no server config is passed to the command
  handler.
• Addressed — Uses AxoUpdater::new_for("torc"), loads the standalone installer receipt, and
  checks that the receipt belongs to std::env::current_exe() before updating. Evidence:
  src/client/commands/self_update.rs calls AxoUpdater::new_for(APP_NAME), load_receipt(), and
  check_receipt_is_for_this_executable() before run_sync().
• Addressed — Supports an optional target release tag/version and a GitHub token via --token
  and GITHUB_TOKEN. Evidence: SelfUpdateArgs has optional TARGET_VERSION and
  #[arg(long, env = "GITHUB_TOKEN")]; tests cover tag vs plain semver target routing.
• Addressed — Missing or mismatched receipt exits non-zero and explains that self-update is only
  available for standalone installer installs. Evidence: missing-receipt unit test and manual CLI
  command return exit 1 with the standalone installer requirement.
• Addressed — Unsupported installs get useful guidance for cargo/crates.io, package-manager,
  Docker, site-managed, and manual archive installs. Evidence: unsupported_install_guidance() and
  missing-receipt test assert the guidance categories.
• Addressed — GitHub API rate-limit failures recommend using a token instead of surfacing an
  opaque error. Evidence: is_github_rate_limit() maps unauthenticated 403/429 reqwest errors to a
  token-focused message.
• Addressed — Tests cover at least missing-receipt behavior and CLI help output. Evidence:
  cargo +1.95.0 test self_update runs 5 passing tests including missing receipt and help output.

Deviations from the original plan

• No release workflow or installer publishing changes are included. Current repository releases ship
  archives, not cargo-dist installer assets/receipts, so actual successful self-update requires a
  standalone installer distribution path to exist. This should stay as a follow-up if desired.
• Used cargo +1.95.0 locally because the active default toolchain was Rust 1.92 while the crate now
  requires Rust 1.95.

Testing Strategy

cargo +1.95.0 test self_update
cargo +1.95.0 run --quiet --bin torc -- self update --help
tmpdir=$(mktemp -d); AXOUPDATER_CONFIG_PATH="$tmpdir" cargo +1.95.0 run --quiet --bin torc -- self update; status=$?; rm -rf "$tmpdir"; test "$status" -ne 0
cargo +1.95.0 fmt --all -- --check
DATABASE_URL=sqlite:db/sqlite/dev.db sqlx database setup --source torc-server/migrations
cargo +1.95.0 clippy --all --all-targets --all-features -- -D warnings
PATH="$HOME/.dprint/bin:$PATH" dprint check

References

• feat: add torc self update command #379
• feat: add torc self update command #379