feat: add Copilot Chat session parser for recall

[bilersan]

Summary

Add a JSONL parser that discovers and parses VS Code Copilot Chat session files, enabling ctx recall to surface conversations from GitHub Copilot alongside existing Claude Code sessions.

Closes #28

Merge Order

Merge first — this PR is standalone with no dependencies on other open PRs.
Suggested order: #29 → #31 → #36

Changes

New files

• internal/recall/parser/copilot.go (522 lines) - CopilotParser implementing the Parser interface with Matches(), ParseFile(), and CopilotSessionDirs() for platform-aware directory discovery
• internal/recall/parser/copilot_raw.go (95 lines) - Typed Go structs for Copilot Chat JSONL format

Modified files

• internal/config/file.go (+2 lines) - Added ToolCopilot = "copilot" constant
• internal/recall/parser/parser.go (+1 line) - Registered NewCopilotParser() in registeredParsers
• internal/recall/parser/query.go (+5 lines) - Added CopilotSessionDirs() to the directory scan loop

How it works

Copilot Chat stores conversations as JSONL files using a snapshot+patch model:

• Kind 0: Full conversation snapshot
• Kind 1: Incremental patch (add message)
• Kind 2: Incremental patch (modify message)

The parser reads the JSONL file, applies patches in order to reconstruct the final conversation state, then builds Session objects with proper metadata (tool, model, timestamp, token counts).

Testing

• All existing parser tests continue to pass
• Copilot-specific tests pass (TestCopilot*, TestScanDir*, TestSession*, TestParse*)
• go build ./... passes
• Compliance tests pass

Notes

• This is a purely additive change — no existing behavior is modified
• The pre-existing TestGetPathRelativeToHome failure on Windows (hardcoded / separator) is NOT introduced by this PR; it exists on upstream/main already