docs: sync documentation with recent code changes

[aheritier]

Syncs docs with code changes merged into main in the last 36 hours.

Changes

PR #3623 — remote MCP header templates and OAuth credential fallback

• docs/features/remote-mcp/index.md: updated config example to use ${env.MY_TOKEN}, added Per-request header template expansion note documenting ${env.VAR} and ${headers.NAME} support; updated the DCR fallback section to describe the new interactive client-credential prompt when DCR is unavailable and no oauth.clientId is configured.
• docs/configuration/tools/index.md: expanded the remote.headers property description with placeholder details.
• docs/tools/mcp/index.md: expanded the remote.headers property description with a link to the detailed note.

PR #3565 — MCP prompts as slash commands

• docs/tools/mcp/index.md: added a new MCP Prompts section documenting that MCP prompt definitions (from the server's /prompts endpoint) are registered as slash commands in the TUI, with usage examples and argument-forwarding rules.

PRs reviewed (no doc changes needed)

• Fix leak connections when fetching JWT token #3633 — JWT connection leak fix (internal)
• fix(tui): wrap long elicitation field content #3628 — TUI elicitation text wrapping (internal UI fix)
• chore: bump direct Go module dependencies #3627 — Go module dependency bumps (chore)
• test: raise input and protocol package coverage #3626 — test coverage increase (test-only)
• fix(mcp): clean stale gateway temp files #3622 — gateway temp file cleanup (internal)
• refactor: adopt pkg/atomicfile for atomic writes #3621 — atomic writes refactor (internal)
• fix(server): limit API request body size #3620 — API request body limit 1 MiB (internal)
• fix(rag): honor ctx cancellation in fan-in collectors #3619 — RAG context cancellation fix (internal)
• fix(tui): queue pending attention events FIFO instead of a single slot (#3584) #3618 — TUI event queue ordering fix (internal)
• chore(runtime): remove dead session event retry #3617 — dead session event retry removed (internal)
• feat(tui): add session path visibility setting #3616 — session path visibility setting (PR included docs)
• fix(eval): stop running containers on Ctrl-C #3615 — eval Ctrl-C container stop (bug fix, expected behavior)
• feat(cli): add "docker agent toolsets" to list built-in toolset types #3614 — docker agent toolsets command (PR included docs)
• docs: document agent delegation feedback UI (transfer and return animations) #3611 — delegation feedback UI (docs-only PR)
• feat(leantui): let embedders override the welcome banner #3610 — lean TUI custom banner (embedder-only Go API, no existing doc section)
• fix(runtime): guard onToolsChanged like onBackgroundEvent (#3594) #3609 — runtime race fix (internal)
• fix(runtime): make startupInfoEmitted atomic (#3593) #3608 — startup info atomic (internal)
• fix(session): make InMemorySessionStore.messageID atomic (#3592) #3607 — session message ID atomic (internal)
• fix(session,server): lock-safe Session.Messages access + 409-busy guard (#3590) #3606 — session lock + 409 guard (API docs already covered)
• refactor(server): decouple WorkingDir from RuntimeConfig in SessionManager #3602 — WorkingDir decoupling (internal refactor)
• fix(config,server): reject zero-agent configs & guard GET /api/agents (#3588) #3589 — zero-agent config rejection (docs already state requirement)
• fix(tui): wire MCP prompts into slash command dispatch #3565 — MCP prompts slash dispatch (documented above)