tools: split filesystem into read, write and file_ai ...

- filesystem_read: read, list, find, find_up, grep, info, exists, diff
- filesystem_write: write, append, edit, edit_lines, insert, mkdir, move, copy, delete
- file_ai: ask (was query), edit (was smart_edit)
- paged numbered reads (500 lines / 50 KB), raw copy mode, binary refusal
- nearest-match and numbered-copy hints on failed edits, context after edit
- guards: empty overwrite, non-empty delete, move/copy overwrite
- cross-tool redirects instead of silent wrong-action errors
- legacy 'filesystem' stays for archived sessions (compression, renderers, webclient)
- profiles, prompts, docs and manuals updated; TUI and webclient render the new tools

core: split the agent and context builder into packages ...

Agent becomes one class assembled from mixins: runtime (turn lifecycle),
streaming (the turn loop), tools (tool execution), compression (context
gates) and context (tool lists, backend lookup, window). The module-level
helpers and sentinels move to helpers.py.

ContextBuilder splits into helpers (pure truncation/freshness/prompt
blocks) and builder (the class). Both old modules become packages with
the same import surface, so navi.core.agent.Agent and
navi.core.context_builder.ContextBuilder keep working.

Method bodies are unchanged; tests now patch settings/log/should_compress
on the mixin that actually reads them.