feat: add shell interface, registry, and bash shell emulator

Implement Phase 1.4: replaces the hardcoded banner/timeout stub with a
proper shell system. Adds a Shell interface with weighted registry for
shell selection, a RecordingChannel wrapper (pass-through for now, prep
for Phase 2.3 replay), and a bash-like shell with fake filesystem,
terminal line reader, and command handling (pwd, ls, cd, cat, whoami,
hostname, id, uname, exit). Sessions now log command/output pairs to
the store and record the shell name.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -37,6 +37,9 @@ Key settings:
 - `storage.db_path` — SQLite database path (default `oubliette.db`)
 - `storage.retention_days` — auto-prune records older than N days (default `90`)
 - `storage.retention_interval` — how often to run retention (default `1h`)
+- `shell.hostname` — hostname shown in shell prompts (default `ubuntu-server`)
+- `shell.banner` — banner displayed on connection
+- `shell.fake_user` — override username in prompt; empty uses the authenticated user
 
 ### Run