feat: add per-session LLM token budget with hard cap and soft warn

[harrykamboj1]

Summary

• Per-session token budget in KV (mem:session-budget), default cap 100k via AGENTMEMORY_SESSION_TOKEN_CAP
• Enforcement at ResilientProvider: block when exhausted, record estimated tokens in finally
• Soft warn at 80% (event::mem::budget::soft-warned); hard cap (event::mem::budget::exhausted)
• Compress → synthetic fallback; summarize → truncated partial summary
• GET /agentmemory/session/budget; agentmemory status shows active / near-cap / exhausted
• OTEL histogram session.tokens_used on meter agentmemory

Closes #767

Test plan

• npm test -- test/session-budget.test.ts (15 tests)
• npm test -- test/consistency.test.ts
• Manual: AGENTMEMORY_AUTO_COMPRESS=true, low cap, verify synthetic fallback + status line

Spec alignment / open items

Gap	Status	Notes
KV shape (5 vs 11 fields)	Intentional superset	Spec minimum + ops fields (inputTokens, callCount, timestamps)
Function count (2 vs 4)	Open	record/reap are implementation; get duplicates REST — can drop if preferred
Reaper: cron vs setInterval	Follows repo pattern	Matches recent-searches-sweep; no cron triggers in repo today
ALS on observe / consolidate-pipeline	Deferred	Observe doesn't call provider; consolidate bills __system__ sentinel
Sentinel unknown vs __system__	Open	Used __system__ for clarity — rename if spec is strict
Atomic increment via kv.update	Kept keyed lock	No increment op used elsewhere in codebase
Cost at display time vs record	Deferred	Flat costEstimate at record for v1
OTEL export name	Verify	Instrument session.tokens_used on meter agentmemory

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