docs: record the Result-handling convention

[epodivilov]

Closes WTK-33.

What changed

• src/cli/commands/create.ts — the git.removeWorktree call inside the CleanupHandle rollback callback now carries an // ignored: <reason> comment. Comment only, no behavior change.
• CLAUDE.md — new "Result handling" subsection under Conventions: Results from port calls must be checked, deliberate exceptions are marked // ignored: <reason>, and mechanical enforcement is deferred.

Decision

No enforcement tooling is being adopted. Biome 2.x GritQL carries no type information, so a syntactic rule would have to match on a hand-maintained list of ~42 port method names and would drift as ports change. Adding a second, type-aware linter to the toolchain is disproportionate for a bug class that currently has zero live instances. The convention is enforced by review, and the // ignored: marker exists so review can distinguish intent from oversight.

Scan

A fresh scan (statement-position port calls, plus assigned-but-never-checked Result variables) finds the create rollback as the only discarded port Result in src/.

Three use-case level functions that return Result are also discarded at their call sites, all deliberately, and none carry the marker yet:

• src/application/use-cases/check-for-updates.ts — writeUpdateCache inside refreshUpdateCache
• src/cli/commands/self-update.ts — writeUpdateCache after a successful update
• src/application/use-cases/update-worktrees.ts — runHooks for on-conflict hooks

Out of scope here; worth a follow-up if the marker should cover use-case Results too.

Checks

pnpm typecheck, pnpm lint, pnpm test — all pass (513 tests, 0 failures).