refactor(hooks/builtins): one file per builtin + simplify registration

[dgageot]

Small readability/maintainability refactor of pkg/hooks/builtins. No
public API change, no behaviour change, all existing tests pass as-is.

What changed

1. Split each builtin into its own file

builtins.go was a 130-line file mixing registration plumbing with three
unrelated builtin implementations. Now each builtin lives in its own file:

• add_date.go — AddDate constant + addDate function
• add_environment_info.go — AddEnvironmentInfo constant + addEnvironmentInfo function
• add_prompt_files.go — AddPromptFiles constant + addPromptFiles function

Each file is now fully self-contained: registered name and behaviour
sit next to each other. builtins.go keeps only the package doc and the
generic plumbing: Register, AgentDefaults, ApplyAgentDefaults, and
the small builtinHook / turnStartContext helpers.

Adding a fourth builtin is now a clear mechanical change: drop a new
add_*.go file with const + func, plus one line in Register.

2. Simplify Register and ApplyAgentDefaults

• Register previously iterated a map[string]BuiltinFunc{...}
  and wrapped each error with fmt.Errorf("register %q builtin: %w", ...).
  The wrapping wasn't reachable in practice (RegisterBuiltin only fails
  on empty name / nil fn, neither possible with package constants), and
  map iteration is non-deterministic. Replaced with three explicit
  RegisterBuiltin calls combined via errors.Join — shorter, ordered,
  and reads top-to-bottom.

• ApplyAgentDefaults had three near-identical 4-line struct
  literals (hooks.Hook{Type: HookTypeBuiltin, Command: ...}). A tiny
  builtinHook(name, args...) helper turns each branch into a
  one-liner, halving the function's size and aligning the three
  flag-handling branches visually.

• Dropped the duplicated "turn_start vs session_start" rationale
  paragraph from ApplyAgentDefaults' doc — it was already in the
  package comment verbatim.

Validation

Rebased on latest origin/main. Repo-wide clean:

• go build ./... ✅
• go vet ./... ✅
• golangci-lint run ./... → 0 issues ✅
• go test ./... → all packages pass ✅
• Existing pkg/hooks/builtins tests pass unchanged (no test edits
  needed — public API is identical).

Out of scope

• AgentDefaults.IsZero() is currently only consumed by its own test.
  Left in place since it's a documented part of the public API and the
  brief said "don't remove any feature."

Assisted-By: docker-agent