fix(cli): preserve literal fill arguments

[yashranaway]

Closes #17.

What changed

• Recognize global --json and --session only before the first -- sentinel.
• Remove the sentinel before command parsing so literal flag-looking values reach fill unchanged.
• Require fill text as one shell argument instead of joining tokens and normalizing whitespace.
• Document the CLI contract in architecture decision 16, help text, and the agent skill.
• Mark backlog A6 complete and add the unreleased changelog entry.

Regression coverage

The protocol suite now checks:

• global options before the sentinel still work;
• literal --json and --session values after the sentinel survive;
• tabs and double spaces are preserved exactly;
• unquoted multi-token fill text is rejected instead of silently normalized.

Verification

• Protocol/security suite in swift:6.1-bookworm: 31/31 passed.
• pnpm test:runtime: passed.
• git diff --check: passed.

The host and wire protocol are unchanged, so no platform E2E label is required.