docs: update docs for --lean TUI, --session new-id, and hook agent_name

[aheritier]

Documentation updates

This PR brings the /docs directory in sync with three PRs merged to main in the last 36 hours.

Changes

Source PR	Doc change
#3119 Support --session new-id	Updated --session flag description to document that an explicit unknown ID creates a new session instead of failing
#3127 Secondary, non-alternate screen ui	Expanded --lean flag description; added callout tip explaining the non-alternate-screen rendering
#3110 feat(hooks): populate agent_name on tool-use hook events	Added agent_name to pre_tool_use, post_tool_use, and permission_request in the hook event fields table; added explanatory note for multi-agent setups

PRs reviewed and found up to date

Source PR	Reason
#3135 fix(acp): respect workspace roots for Zed paths	Internal ACP path-resolution fix; no user-facing doc impact
#3133 Disable old tool-call truncation by default	Docs already updated in the PR
#3131 feat: warn on unpinned external agent refs and document digest pinning	Docs already updated in the PR
#3130 fix(skills): discover .agents/skills in non-git grouping parents	Internal discovery behavior fix; no doc-visible contract change
#3129 fix(runtime): finalize event channel on runStreamLoop early returns	Internal runtime fix; no user-facing behavior change
#3128 perf(tui): skip sidebar scrollbar-probe pass on animation frames	Internal TUI perf optimization; no doc needed
#3126 fix(tui): cache sidebar todo render	Internal TUI perf optimization; no doc needed
#3125 fix: run /shell in the active session's working directory	Bug fix; behavior now matches existing doc expectation
#3124 Improve the TUI control API	Docs already updated in the PR
#3120 Better worktree handling	Docs already updated in the PR
#3113 fix: remove todo strikethrough	Cosmetic UI-only change; no doc impact
#3111 docs: fix per-model Shift+Tab thinking-level cycle description	Docs-only PR; already correct