Feature: add "Copy as Markdown" context menu action (#21)

[Bhumika-1432006]

Fixes #21

Feature

Tree items already carry a contextValue (treeProvider.ts, set to the pass/fail/unknown status), but no context menu was ever wired up to use it. Right-clicking a history entry did nothing TruthLog-specific — you had to click the row, wait for the read-only editor tab to open, select all, copy, then switch back and paste. This adds a "Copy as Markdown" right-click action that collapses that into one click, matching the extension's own stated purpose: giving you ground truth to paste back into a conversation with an AI agent when it misreports a result.

Implementation

• extension.ts: added renderAsMarkdown(execution), which wraps the existing renderOutputDocument() header+output formatting (already used for the "click to open output" viewer) in a fenced markdown code block — no new formatting logic invented, just reused and wrapped so it pastes cleanly into a markdown-rendering chat. Registered a new command, truthlog.copyAsMarkdown, that resolves the target execution and writes the rendered markdown via vscode.env.clipboard.writeText.
• Added executionIdFromCommandArg() to accept either a bare execution id (string) or the full Execution-shaped object. This matters because VS Code invokes a view/item/context command with the selected tree element itself as the argument (the actual Execution object, since TruthLogTreeProvider is a TreeDataProvider<Execution>) — unlike truthlog.openOutput, which is wired to a manually-specified id via item.command.arguments. Falls back gracefully (logs a warning, no-op) if the argument doesn't resolve to a valid id, and shows a warning message if the execution is no longer in history (cleared or trimmed) — mirroring the existing fallback text in OutputFileSystemProvider.readFile.
• package.json: added the truthlog.copyAsMarkdown command declaration (with a $(copy) icon), hid it from the command palette (when: "false", matching truthlog.openOutput's existing pattern, since it needs a selected item to make sense), and added a contributes.menus.view/item/context entry keyed off view == truthlog.history so it shows up in the right-click menu for history rows.

No architecture changes, no new state, no new settings.

Testing

• npm run compile passes cleanly with no errors.
• Added two tests to src/test/suite/recording.test.ts, following the existing pattern used for truthlog.openOutput:
  • invokes truthlog.copyAsMarkdown with the full Execution object (matching how a real context-menu click invokes it) and asserts the clipboard contains a properly-fenced (```-delimited) block including the command text, the exit code line, and the raw output
  • invokes it with a bare execution id string, asserting the dual argument-shape support works
• Additionally sanity-checked the exact renderAsMarkdown/renderOutputDocument formatting logic in a standalone plain-Node script (bypassing the blocked Electron host) — confirmed the fence markers, command text, exit-code line, and marker output all appear exactly as the new tests assert.
• Honest caveat, same as Fix: diagnostic notes silently dropped once output is already truncated (#15) #16, Fix: endSub has no timeout on readDone, unlike closeSub (#17) #18, and Fix: ExecutionStore trusts persisted workspaceState data with zero validation (#19) #20: I could not run npm test (the Electron-hosted VS Code integration suite) in this environment — same Windows-sandbox limitation as those PRs (Code.exe doesn't launch as a real GUI app here, matching why this repo's own CI only runs the integration suite on ubuntu-latest under xvfb). This PR's CI run will be the first real execution of the new tests.