feat(cli): add /tag for multi-label session filtering

[emal-avala]

Summary

Adds /tag — multi-label session categorization that complements /rename (which is a single human-readable label). Tags let users filter /sessions by project / feature / mood / whatever.

Commands:

/tag                   list tags on the current session
/tag <tag>             add a tag (idempotent)
/tag --remove <tag>    remove one
/tag --clear           remove all
/sessions --tag <tag>  filter the session list

Tag normalization (services::session::normalize_tag):

• lowercase, trimmed
• alphanumeric + - / _ only
• max 32 chars

Rejected tags get a clear error — no path-escape or shell-metachar leakage.

Implementation

• SessionData.tags: Vec<String> with #[serde(default)] — existing session files load without tags and default to empty
• add_session_tag / remove_session_tag use the same load-modify-save path as set_session_label, so the per-turn save hot path is untouched
• SessionSummary.tags surfaces in /sessions as #tag1 #tag2 inline
• /sessions --tag <t> retains only matching entries; invalid tag arg aborts with a clear error

Test plan

• cargo fmt --all clean
• cargo check passes
• cargo clippy --no-deps clean
• cargo test -p agent-code-lib --lib session — 21 pass, 3 new (normalize_tag accept/reject + old-JSON defaults-to-empty)
• Manual: /tag wip then /sessions --tag wip shows only the current session

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