refactor: table-drive source shorthand and single-id subcommand wiring in cmd/ccsession

[sorafujitani]

Context

cmd/ccsession/main.go (548 lines) repeats the same wiring blocks per backend and per subcommand; each new backend or id-taking subcommand adds another copy.

Current state

• applySource (cmd/ccsession/main.go:227-263): four near-identical "conflict check → assign name" blocks for all/opencode/grok/codex.
• parseGlobalFlags (main.go:277-303): four identical bool-sugar blocks for --all/--opencode/--grok/--codex.
• cmdResumeSpec and cmdResume (main.go:379-436): structurally identical "one --locator flag, exactly one positional id" scaffolding.

(Line numbers as of e5bc967 — re-locate with grep -n if drifted.)

Task

1. Replace the shorthand blocks with a table:

   var sourceShorthands = map[string]string{
       "--all": "all", "--opencode": "opencode", "--grok": "grok", "--codex": "codex",
   }

   and one loop that performs the conflict check ("cannot combine --X with --source/--Y") with the same error strings as today. Grep main_test.go for the exact expected messages before touching them.
2. Extract the shared subcommand scaffolding:

   func runIDCommand(name string, args []string, fn func(id, locator string) error) error

   and reimplement cmdResumeSpec/cmdResume (and cmdPreview's overlapping parts if it fits naturally — don't force it) on top of it.
3. Behavior must be byte-identical, including usage/help text and error messages. cmd/ccsession/main_test.go already covers much of this; extend it for any conflict combination not yet asserted.

Acceptance criteria

• Adding a hypothetical new backend shorthand requires touching one table, not two function bodies.
• All existing tests pass unmodified except where they gain coverage; go vet, golangci-lint run, gofmt -l . clean.