feat: add context placeholders for ralph name, iteration, and max_iterations

[malpou]

Summary

Closes #14.

• Adds {{ context.name }}, {{ context.iteration }}, and {{ context.max_iterations }} built-in placeholders so ralphs can access runtime metadata in their prompts
• No frontmatter configuration needed — these are automatically available
• context.name resolves to the ralph directory name, context.iteration to the 1-based iteration number, and context.max_iterations to the total if -n was set (empty otherwise)

Changes

• _frontmatter.py — new FIELD_CONTEXT constant
• _resolver.py — extended _ALL_PATTERN regex and resolve_all() to handle the context namespace
• engine.py — new _build_context() helper, updated _assemble_prompt() signature to accept RunState
• 6 new tests covering context resolution, missing keys, mixed namespaces, and engine integration
• Docs updated: writing-prompts, quick-reference, codebase-map, SKILL.md, changelog (0.2.5)

Test plan

• All 442 tests pass (uv run pytest)
• Manual: create a ralph using {{ context.iteration }} in the prompt body and verify it resolves to the correct iteration number

---
agent: cat
---

Iteration {{ context.iteration }} of {{ context.max_iterations }}.
Ralph name is {{ context.name }}.

• Manual: verify {{ context.max_iterations }} is empty when -n is not set

---
agent: cat
---

Hello from {{ context.name }}!
This is iteration {{ context.iteration }}.
Max iterations: [{{ context.max_iterations }}]

[kasperjunge]

Thanks for this one — clean implementation and great test coverage.

One naming change I'd like before merging: rename context.* to ralph.*. So {{ ralph.name }}, {{ ralph.iteration }}, {{ ralph.max_iterations }}.

Reasoning: in a RALPH.md file, {{ ralph.name }} is immediately self-documenting — "this ralph's name." context is an overloaded term that requires a mental lookup ("context of what?"). ralph.* reads naturally and there's zero ambiguity.

If you update the namespace naming + docs, this is good to merge.

[kasperjunge]

Superseded by #37, which renames context.* to ralph.* as discussed. Thanks @malpou for the original implementation — your commits are preserved with authorship in the new PR.